The invention relates to a process for testing returnable bottles which have been sent back for refilling, in particular PET bottles, for the presence of contamination by testing gas samples taken from individual bottles as the bottles are transported along a conveyor path. The invention also relates to an apparatus for carrying out the process.
For returnable bottles, in particular plastic bottles such as PET bottles, which cannot be washed at high temperatures, the problem arises that contamination needs to be reliably detected in order that contaminated bottles can be removed and not refilled. In particular it must be possible to detect returned bottles which a user has used for potentially hazardous substances (poisons, solvents, etc). An already known procedure is to take a gas sample from each bottle and to analyse the sample by photo-ionization detection (PID). This enables undesired substances present even in small traces in the bottle and/or in the plastic material to be detected. As such testing devices need to have a high throughput of bottles per minute (say 250 to 300 bottles per minute) to enable this kind of testing to be undertaken in an industrial bottling plant, a large number of individual PID units have hitherto been used, with each of a number of bottles to be tested having an individual PID unit assigned to it. This involves high costs in terms of money and maintenance and calibration time.
It is therefore a desirable object in itself to reduce the number of test units. But if there is to be no reduction in the bottle throughput, it is necessary for the measuring time per bottle to be shortened, because with a reduced number of test units each unit will have to test more than one bottle in a given period of time. It must particularly be borne in mind, however, that not only is the actual measuring time for the individual gas sample in the test apparatus significant, but also allowance must be made for a recovery time which the test apparatus requires after heavy contamination has been detected in order to regain sufficient sensitivity to detect a low level of contamination; this phenomenon is sometimes called the "memory effect". Such an effect occurs in all suitable types of testing apparatus for the gas samples, including the PID test apparatus already mentioned, and including mass spectrometers.